Digitizer tablets are used in conjunction with a writing pen or instrument for converting graphic information into digital coordinate signals. The tablet or platen is formed with an XY grid of parallel conductors to which signals are applied for detection by the pen or instrument or which are scanned to detect signals from the pen or instrument. A variety of known methods are available for converting the location and movement information of the pen or instrument relative to the tablet into electrical signals such as, for example, described in U.S. Pat. Nos. 3,767,858; 3,983,322; 4,022,971 and 4,185,165.
A typical digitizer or platen has first and second sets of grid conductors in parallel planes embedded in a resin plate having a flat work surface referred to as the digitizing surface. The conductors of the first grid run perpendicular to the conductors of the second grid and all the grid conductors are electrically insulated from each other. The writing pen or other instrument bears upon the digitizing or working surface.
Prior tablets or platens of a resin material are typically manufactured by pulling grid wire material such as 0.02 inch piano wire or music wire, a structurally strong steel wire, between precision spacing guides or bridges. The piano wire is cut to length and loops are formed at each end. Tension is applied to each individual wire aligned over a cavity, using, for example, springs to hold them tightly. In this manner, two sets of conductors in the X and Y coordinate directions are tautly suspended in planes spaced from each other approximately 0.05 inches apart. The taut X and Y conductors extend through a mold or cavity and a rubber gasketed cover is placed over the wire matrix. Liquid resins such as liquid vinyl resin or catalyzed polyester resin are introduced into the cavity or mold. After the resin has been poured and the mold or cavity filled so that the X and Y coordinate grids are immersed and embedded in the liquid, the resin is allowed to cure for approximately 14 hours at a temperature of approximately 75 degrees F. The exotherm temperature of the resin at the time of curing may reach 200 degrees F., however.
A number of disadvantages are attendant upon this conventional method for fabrication of digitizer tablets and platens. It is difficult to apply sufficient tension to the highly tensile piano wire to straighten out kinks or bends in the wire and the taut music wire presents a danger to workers. It is costly to cut the wires individually to length and form loops and highly accurate positioning of the individual wires is difficult to achieve. Furthermore, during curing the polyester resin shifts, shrinks, expands and distorts the wire matrix grid pattern. The displaced grid conductors do not return precisely to their original position thereby limiting the accuracy of the grid conductors for encoding graphic position information.
Another method of fabricating digitizer tablets or platens uses screen printing and printed circuit techniques for forming the grids of parallel conductor strips on first and second glass plates. As described in U.S. Pat. No. 4,255,617, the X and Y coordinate grids are formed respectively on two pieces of float glass as shown in FIG. 3 of that patent, and the two pieces of glass are laminated together by a resin layer that separates and insulates the orthogonal X and Y grid components. Such a method, however, suffers the disadvantage that multiple glass plates must be used, and the printed circuit traces are subject to damage or defect during manufacture in contrast with the wire grid matrix.
U.S. Pat. No. 2,194,551 describes a method for producing a polarizing body consisting of a glass plate with fine wires embedded in the glass to a density of, for example, 40,000 or more parallel layers per inch. According to the disclosure in this patent, a method is proposed for packing powdered glass above and below the mounted wires and heating the whole mass in a furnace so that the glass and wires are heated simultaneously, the glass melting around the wires. The glass and wire are then stretched together while the glass is in a plastic state. The stretching elongates the glass and wires, bringing the wires closer together for light polarizing effects. This U.S. patent disclosure is for an entirely different purpose from the present invention, actually heating and stretching both a glass material and embedded set of wires together.
U.S. Pat. No. 2,194,551 relates only to methods of producing flat surfaces such as mesh screens or screen electrodes by a stretching treatment and is otherwise unrelated to the present invention.